The present invention relates to an error control apparatus and method for a digital transmission system such as a General Packet Radio Services system (GPRS) having a transmission medium for performing transmission in a first and a second direction.
In a digital transmission system, transmission errors occur due to noise and distortion. There are two types of transmission errors: random errors and burst errors. Random errors may be caused by thermal noise. Burst errors are generated during a fade in the transmission channel.
Transmission errors can be detected by adding a redundant signal (check bits) to the information signal. Upon detecting a transmission error, there are two ways to control the error. One is called ARQ (Automatic Repeat Request) and uses a feedback control to request a retransmission of the corrupted data. The other, called FEC (Feed-Forward Error Correction), uses a feed-forward control to control the errors.
A system employing an ARQ scheme requests a retransmission of the data received in error. There are three ARQ strategies: (i) Stop-and-Wait (ii) Go-Back-N, and (iii) Selective Repeat.
In the Stop-and-Wait ARQ system, the receiver sends an ACK signal to acknowledge a correct reception, and sends a NAK signal to request a retransmission of a coded signal block detected in error. However, the efficiency of transmission is low, since a confirmation of the received signal is made for every block of data.
In the Go-Back-N ARQ system, a coded data block is transmitted continuously. If the transmitter receives a NAK signal requesting a retransmission then the transmitter goes back by N data blocks and restarts the transmission from that data block. The time delay between the transmission of a data block and the reception of a NAK signal must be less than the time for the transmission of N data blocks. The idle time is shorter for this system than for the Stop-and-Wait ARQ system. However, it still wastes time, since N data blocks, which may include blocks with error, are transmitted for every retransmission.
In the Selective Repeat ARQ system, data blocks are sent continuously and only the blocks in error are retransmitted. Thus, this system has the highest efficiency. The management of the order of data blocks and the buffer is more complicated for this system compared to the other systems.
Moreover, a method is known where an erroneous block is stored and used as a diversity when retransmission occurs. However, a memory is required for the erroneous blocks.
There are other techniques, known as hybrid ARQ systems, wherein a combination of an ARQ scheme and a forward error correction is used. In this context a type II hybrid ARQ is known, wherein a data block is transmitted at first in a heavily punctured way, and in case of required retransmissions, the punctured bits are transmitted. This results in a good performance in one direction of transmission, but requires quite a lot acknowledge messaging in the other direction, reducing the possible data rate in that direction.
Accordingly, a problem arises in that inoptimization of the data transfer rate in one direction, for example a downlink direction, requires a lot ACK/NAK messaging in the other direction, i.e. the uplink direction. This leads to a reduction of the possible data rate available in the uplink direction to the same or other users.
It is an object of the present invention to provide an error control apparatus and method by means of which the overall data throughput in both transmission directions can be optimized.
This object is achieved by an error control apparatus for a digital transmission system having a transmission medium for performing a transmission in a fist and a second direction, comprising:
error detection means for detecting an error in a digital signal transmitted in said first direction via said transmission medium;
transmitting means for transmitting a control message in said second direction via said transmission medium so as to request a retransmission of said digital signal, if said error detection means detects an error in said digital signal; and
control means for controlling a transmission rate of said control message in dependence on an usage of said transmission medium in said second direction and/or an amount of available memory in a receiving means for receiving said digital signal.
Furthermore, the above object is achieved by an error control method for a transmission system having a transmission medium for performing transmission in a first and a second direction, comprising the steps of:
detecting an error in a digital signal transmitted in said first direction via said transmission medium;
transmitting a control message in said second direction. via said transmission medium so as to request a retransmission of said digital signal if an error has been detected in said error detection step; and
controlling a transmission rate of said control message in dependence on a usage of said transmission medium in said second direction and/or an amount of available memory in a receiving means for receiving said digital signal.
According to the invention, the network capacity can be optimized by controlling the transmission rate of the control message so as to achieve high throughput rates. As an example, the throughput in one direction can be increased by allowing more messaging, i.e. a higher control message rate, in the other direction, if the usage of the other direction is low. When there is a data transmission in both directions, the overall data throughput can be optimized by changing the rate of the control messages in accordance with the usage of the respective transmission directions.
Alternatively or additionally, the transmission rate of the control message can be controlled on the basis of the available memory in the receiver, such that a lower transmission-rate can be used if a larger memory for error processing is available in the receiver.
In case of a diversity ARQ method, an erroneous portion of the digital signal can be stored in a storing means and can be used for the diversity processing after the retransmission has been performed. Thereby, a transmission method with a higher bit rate can be used due to the improved error correction efficiency, so as to increase the throughput rate.
In the above case, the control means can be arranged to adjust the transmission rate in accordance with an available amount of memory in said storing means. Thus, a loss of erroneous signal portions required for the diversity processing can be prevented.
Preferably, the control means can be arranged to reduce the transmission rate and to select another coding scheme which leads to a smaller amount of errors, when a signal transmission is performed in the second transmission direction.
Accordingly, the overall throughput can be optimized by reducing the transmission rate of the control message and selecting a better coding scheme leading to a smaller amount of errors.